Glass ionomer cements have been used for a variety of dental purposes. When introduced, glass ionomer cements were of a consistency rendering them useful as a restorative material. Today, glass ionomer restorative materials are successful in restoring significant lesions. For example, Matis, et al., How Finishing Affects Glass Ionomers, Journal of the American Dental Association, 1991; 122:43-46, describes a five year study to determine the effectiveness of restorations involving the use of glass ionomer restorative materials and concluding that glass ionomer restorative is materials are outstanding in their retentive capability.
Studies have been conducted to determine the capability of glass ionomer cement to adhere to various materials, including the tests disclosed in Holtz, et al., The Bonding of Glass ionomer Cements to Metal and Tooth Substrates, British Dental Journal, 1977; 142:41-47. This study demonstrated that glass ionomer cement bonds well to dentin and enamel, and also adheres to some cast metals. Adherence to cast metals is most successful when the surface of the metal is first etched with an acid, such as citric acid, before applying the glass ionomer cement as such etching enhances the mechanical bond between the cement and the metal.
Results of studies such as Holtz, et al. have provided a basis for the use of glass ionomer cements for the application of metal and porcelain prostheses. For example, in U.S. Pat. No. 4,654,007, a layer of glass ionomer cement is applied to a tooth prior to attaching a porcelain restoration thereto. After the cement is properly hardened and before the porcelain restoration is bonded with an acrylic cement, the hardened glass ionomer cement is etched to create microscopic surface irregularities which facilitate mechanical retention of the acrylic cement to the glass ionomer cement.
Several years after glass ionomer restorative materials were first made available to dentists, new formulations of glass ionomer cements were developed for use as a luting agents or bases or liners. When used as a base or liner for amalgam restorations, glass ionomer cements are first allowed to harden in the tooth before the amalgam is placed in the tooth on the hardened glass ionomer cement. Studies showed that when the glass ionomer cement was hardened that the glass ionomer cement shrinks, leaving a slight 60-80 um gap between the hardened glass ionomer cement and the hardened amalgam. Scherer, Reinforced Glass ionomer Cement vs. Zinc Phosphate Cement, 18th Annual Session of the American Association for Dental Research, San Francisco, California. Thus, hardened glass ionomer cement bases/liners/luting agents do not in and of themselves bond the amalgam to the tooth.
The invention disclosed in U.S. patent application Ser. No. 07/942,375, filed Sep. 9, 1992, the disclosure of which is incorporated herein by reference, comprises a dental restoration system that works well with conventional materials, specifically glass ionomer cement and amalgam, to allow the tooth to be filled instead of being extracted or requiring the application of a prosthesis such as a crown or bridge. Such a system is inexpensive to use and results in an improved bond strength over prior methods without requiring that the glass ionomer cement first be hardened and then acid etched before filling the lesion with amalgam.
Glass ionomer cements, whether used as a restorative material, a cement for a prosthesis such as a bridge or crown, or as a luting agent/base/liner, generally comprise a glass ionomer cement powder which is mixed with a glass ionomer cement liquid. Before application, the powder is mixed with the liquid to form the cement. For all glass ionomer cements, the powder component is generally composed of silica and various oxides and fluorides. For example, the powder component of the glass ionomer Cement disclosed in U.S. Pat. No. Re. 33,100, which is intended for use as a luting agent, base or liner, comprises:
______________________________________ Percentage Composition ______________________________________ 26% SO.sub.2 Silica 6% B.sub.2 O.sub.3 Boron Oxide 16% Al.sub.2 O.sub.3 Aluminum Oxide 6% AlF.sub.3 Aluminum Fluoride 5% NH.sub.4 F Ammonium Fluoride 4% P.sub.2 O.sub.5 Phosphorus Pentoxide 37% CaF.sub.2 Calcium Fluoride ______________________________________
The liquid portion of glass ionomer cement disclosed in U.S. Pat. No. Re. 33,100 comprises:
______________________________________ 91.5% Polyacrylic acid of a low molecular weight, 40.0% solution in water 8.5% D-tartaric acid ______________________________________
The powder portion of the glass ionomer cement is mixed with the liquid portion in a ratio of 1:1 to 2:1 by weight.
Two other glass ionomer cements intended for use as a luting agent, base or liner, are Ketac-CEM Radiopaque glass ionomer cement distributed by ESPE Premier Sales Corporation of Norristown, Pennsylvania, and GlasIonomer Type I cement distributed by Shofu Dental Corporation of Menlo, California. Glass ionomer restorative materials are understood to be of a similar composition, but there are differences in particle sizes, mix ratios, liquid compositions, and formulations which vary materials from manufacturer to manufacturer.
Numerous variations of the composition of glass ionomer cements have been made in an effort to improve one of more characteristics of the cement. An ideal glass ionomer cement has a pleasant color similar to that of tooth structure, is easily mixed, sets in a controlled manner in a reasonable time frame. To accomplish these objectives, the cement must be chemically reactive to tooth structure, strong and insoluble, and must be biocompatible, not caustic. Various degrees of setting time, color and handling characteristics are desired depending on the particular application of the glass ionomer cement. For example, when used as a cement for a crown, bridge or other prosthesis, the glass ionomer should not set quickly to allow full seating of the prosthesis before the cement hardens. Should the cement be used as a base or liner under an amalgam restoration wherein the cement is first allowed to harden, faster setting times are desirable.
Some known additives to glass ionomer cement to control the cement's handling characteristics, color, and setting times include, for example, zinc oxide as disclosed in U.S. Pat. No. Re. 33,100. Zinc oxide as an additive to glass ionomer cement primarily serves as a buffering agent, neutralizing the acidity of the cement to reduce pulpal irritations. The presence of zinc oxide also increases the rate of setting reaction, results in a rougher surface when etched, and controls the setting reaction time. U.S. Pat. No. Re. 33,100 also discloses titanium dioxide as an additive to glass ionomer cement which further increases the cement's strength, further controls the setting reaction, and changes the color of the cement so that it is readily distinguishable from that of tooth structure.
U.S. Pat. No. 4,775,592 suggests treating the surface of a fluoroaluminosilicate glass powder of a glass ionomer cement with fluoride to improve crushing strength and to result in a fluidity or viscosity of a mixed cement which improves workability or mixability. To treat the glass powder, the fluoride is not simply mixed with the glass powder, but rather conventional treatment techniques are employed. For example, the treatment may involve mixing the fluoride with water, mixing the fluoride-water mixture with the glass powder, and evaporating the water content out of the resulting mixture.
Zirconia (zirconium oxide) has also been used as an additive with glass ionomer cement compositions. Japanese patent no. 2,275,731 (WPI 90-380320/51) discloses a glass ionomer cement to which small percentages of zirconium oxide and zinc oxide have been added to improve the cement's resistance to disintegration and crushing and to improve hardening time.
Each of these additives, zinc oxide, zirconia and fluorides, may chemically react with the glass ionomer cement composition. Potentially, each chemical reaction may adversely affect the cement's functionality. Thus, it is desirable to provide an improved glass ionomer cement comprising the conventional glass ionomer cement formula and additives that are non-reactive to the cement. In this manner, the glass ionomer cement is not significantly modified, since the additive does not chemically react with the cement, so as to diminish the capability of the cement to perform the tasks for which it is intended and for which it is used in current applications.
When the glass ionomer cement is used as a base for an amalgam restoration whereby the cement is first allowed to harden prior to the application of amalgam to the lesion, some manufacturers, as mentioned in U.S. Pat. No. Re. 33,100, have added the silver alloy powder component of an amalgam into the powder component of the glass ionomer cement with the intent of strengthening the cement without affecting the cement's adhesive properties. However, the silver contained in the additive corrodes in the oral environment and can turn the tooth dark. Therefore, it is desirable to provide an additive which is not corrosive in the oral environment.
Zircon, also known as zirconium silicate or zirconium silicon oxide, is currently used as a prophylaxis polishing agent. Stookey, et al., Studies Concerning the Polishing Properties of Zirconium Silicate on Enamel, Journal of Peridontal, 1966; 37(3): 200-207. Zirconium silicate has also been added to chewing gum. Anderson, et al., Effects of Zirconium Silicate Chewing Gum on Plaque and Gingivitis, Quintessence International, 1990; 21(6):479-489.
Due to its use as a prophylaxis polishing agent, the biological effects of zirconium silicate on various tissues have been studied. Stookey, et al., Studies Concerning the Biological Properties of Zirconium Silicate, Journal of Peridontal, 1967; 38(1): 53-63. This study indicated that zirconium silicate, when introduced in modest amounts, is non-toxic. It is noted that the reactions, or lack thereof, caused by zirconium silicate is in contrast to other forms of compositions containing the element zirconium. Specifically, reactions within tissue have been documented for some zirconium salts, such as sodium zirconium lactate. Further, the element zirconium has been reported to be retained in the osteoid fraction of bone. Because the tooth does not contain an osteoid fraction, the element zirconium is not necessarily retained within the hard tissues of the tooth. Thus, biologically, zirconium silicate is essentially non-reactive and non-toxic and thus a viable candidate for use in dental materials as is evident from its use as a polishing agent.
It is desired to provide various compositions of glass ionomer cements which are varied in terms of its color, viscosity and setting time to meet the particular application for which the cement is intended. It is also desired to provide glass ionomer cement that may be used as a restorative material, a cement for affixing a prosthesis, or a luting agent, base or liner. It is further desired to provide an additive to glass ionomer cement which may be added to any glass ionomer cement composition for any application in varying amounts to affect the characteristics of the cement without affecting the cement's intended functionality. Further, since some glass ionomer cement additives create a desired result in certain respects, such as the reduction of pupal sensitivity by neutralizing the cement's pH level through the addition of zinc oxide to the cement, but also create undesired results in other respects, it is desired to provide a mechanism by which the consequential affects of such an additive may be neutralized or negated.